The field of the invention generally relates to defrosting refrigerators, and more particularly relates to apparatus and method for defrosting a refrigerator in a vacation mode wherein the interval between defrost cycles or operations is significantly increased from the normal adaptive defrost mode.
As is well known, frost or ice forms on the evaporator coil during normal operation of a refrigerator, and eventually the ice will build up to a level where it significantly interferes with the transfer of heat to the evaporator coil. Accordingly, it has been conventional practice to periodically remove ice from the evaporator coil. Automatic defrost control systems have been used to periodically interrupt normal operation of the refrigerator, and energize a heater coupled to the evaporator coil so as to melt the accumulated ice. It has also been known that the rate at which ice accumulates is a function of a number of factors such as ambient humidity, length and frequency of door openings, and the run time of the compressor. In order to improve the overall efficiency of the refrigerator, the intervals between defrost cycles have been varied according to actual need, and control systems that adapt in this way are generally called adaptive or demand defrost systems. In other words, they execute defrost cycles only when needed, and accordingly they avoid the use energy for unnecessary defrost cycles.
One prior art approach for varying the intervals between defrost cycles is to measure the ice buildup on the evaporator, and then to execute a defrost cycle when it reaches a predetermined level. In order to sense the ice buildup, these systems have used mechanical probes, photo-electric sensors, air flow impedance sensors, and sensors responsive to temperature differences between parts of the refrigeration system. However, it has been found that it is very difficult to accurately measure the buildup of ice using this general approach.
Another prior art approach is called "predictive type systems", and these systems generally take into account such parameters as ambient humidity, refrigerator door openings, and total accumulated compressor running time to predict the rate of frost buildup on the evaporator, and thus the required time interval between successive automatic defrosting operations. One predictive type system is described in U.S. Pat. No. 4,156,350 wherein a demand defrost control bases the interval between future defrosting operations on the time required for the defrost heater to raise the evaporator temperature to a predetermined temperature during a previous defrosting operation. That is, a timer measures the heater ON time from the time it is energized until a bimetallic switch on the evaporator coil reaches a predetermined temperature and interrupts the current through the heater. This defrost time corresponds to the accumulation of ice, and is inversely related to the desired interval until the next defrost cycle is needed. That is, if there were a large accumulation of ice, it would be desirable to defrost again relatively soon; and if there were little accumulation of ice, it would be desirable to wait a relatively long time before defrosting again. Accordingly, this system determines the next interval according to an inverse relationship with the measured heater ON time. However, this system does not adapt well to periods of irregular use such as, for example, when the owners are on vacations for extended periods of time.